In spite of the popular belief that neurons are the main constituents of the human brain, most of brain cells are not actually neurons, but star-shaped glial cells known as astrocytes. Their role up—until recently—was believed to be neuron sustenance, neurotransmitter recycling and maintenance of the blood brain barrier. In recent years, it has become increasingly evident that their role in brain function may be more protagonistic than previously thought.
Hundreds of astrocytes can be connected simultaneously to allow collective metabolic and electric coupling as well as calcium-wave signalling. Such inter-astrocyte communication is attained by sharing cytoplasmic content through special channels called gap junctions channels, each formed by two hemichannels contributed by each adjacent cell. Each hemichannel is composed of six protein subunits termed connexins (Cxs), with connexin 43 (Cx43) and connexin 30 (Cx30) as the main connexin constituents of astrocytes and Cx43 being the most abundantly expressed. Interestingly, hemichannels do not always form gap junction channels and may also be found in non-contacting membranes where they allow intracellular-extracellular communication. Although such “functional hemichannels” show low open probability in cultured cells under resting conditions, when opened, they may allow the release of neuroactive substances such as ATP(1) and glutamate(1,2) and several others, now known as “gliotransmitters”. Astrocytes also express another type of hemichannels known as pannexins, particularly pannexin 1(1,3). In vivo, the location of Cx43, Cx30 and pannexin 1 hemichannels at the unopposed surface of astrocytes leaves them in a unique position to mediate the release of gliotransmitters into the chemical neuronal synapse.
Recently, an increasing number of studies have supported the notion that synapses between two neurons may require signals released from an astrocyte to be fully functional(4). Further evidence from in vitro studies indicates that release of gliotransmitters from astrocytes is required to attain synaptic plasticity(5) including release of D-serine(6,7), glutamate(2,8) and ATP(1,9). Moreover, glutamate release from astrocytes may also affect neurotransmitter release at synapses(10), suggesting that presynaptic efficacy and postsynaptic responses are both modulated by astrocytic release of gliotransmitters. The mechanism by which neuroactive gliotransmitters are released remains unknown, although vesicles, P2X7 channels, transporters(10) and Cx hemichannels(1,9) have been proposed. However, there is to date no in vivo evidence for the release of gliotransmitters from astrocytes and no studies so far have shown that Cx43, Cx30 and Panx1 hemichannels are involved in gliotransmitter release in vivo. The lack of such evidence can be at least in part attributed to a lack of tools that affect gliotransmitter release from astrocytes without affecting neuronal synaptic release or inter astrocyte communication. These types of compounds could be used for modulating release of substances from astrocytes to synaptic space, without affecting inter astrocyte communication, thus turning astrocytes into pharmacological targets for treating psychiatric disorders.
Samoilova et al., 2008(12) studied the use of connexin mimetic peptides corresponding to aminoacidic sequences and the general gap junctional blocker, carbenoxolone in generation of epileptiform activity. They postulate that gap junctions can modulate brain activity using rat organotypic hippocampal slice cultures. Samoilova et al., 2008 describes the use of compounds for blocking connexin gap junctions and not connexin hemichannels as the present invention and also does not mention connexin hemichannels from astrocytes as pharmacological target, nor psychiatric disorders for their use.
Danesh-Meyer et al., 2012(13) studied the use of connexin 43 mimetic peptides to investigate neuronal rescue following retinal ischaemia-reperfusion and vascular permeability by transiently blocking connexin 43 activity. They postulate blocking of connexin 43 gap junctions as pharmacological target for the treatment of central nervous system ischemia. Danesh-Meyer et al., 2012 describes the use of compounds for blocking connexin gap junctions from astrocytes but does not mention connexin hemichannels from the cells for the treatment of psychiatric disorders.
Thomson et al., 2008(14) studied the use of specific inhibitors of pannexin 1 from pyramidal neurons in epileptiform seizure activity. They postulate that pannexin 1 hemichannel contributes to epileptiform activity, turning pannexin 1 as a pharmacological target for treatment of epilepsy. Thomson et al., 2008 does not mention pannexins from astrocytes, nor Cx43 or Cx30 hemichannels, only pannexin 1 from pyramidal neurons, thus, they do not postulate astrocytes as pharmacological targets for psychiatric disorders.
Orellana et al., 2009(15) studied modulation of glial connexin and pannexin hemichannels and gap junctions and the effect of their activation in inflammation and neurodegeneration. Orellana et al., 2009 postulates the use of modulation of connexin and pannexin hemichannels and gap junctions for treatment of neurodegeneration, but does not postulate specifically connexin and pannexin hemichannels from astrocytes as a pharmacological target for psychiatric disorders; they are focused on complete CNS. Furthermore, this document also fails to mention the differential modulation, affecting hemichannels but not gap junctions.
Kawasaki et al., 2009(16) discloses that in astrocytes of a Parkinson's disease model, phosphorylated connexin is increased. Phosphorylated connexin is necessary for forming gap junctions, and the authors postulate that modulation of connexin 43 could be a pharmacological target for Parkinson's disease treatment. Kawasaki et al., 2009 is focused on connexin gap junctions, and not connexin hemichannels, as the present invention, and does not mention psychiatric disorders.
Behrens et al., 2011(17) studied the effect of a non-specific blocker of connexin 36, 43 and 50 gap junctions in epileptiform activity. It reported that mefloquine increases hyperpolarization of hippocampal pyramidal cells and promotes action potentials in epileptiform activity. Behrens et al., 2011 postulates connexin gap junctions from pyramidal neurons as a pharmacological target for the treatment of epilepsy, but does not include connexin hemichannels from astrocytes, nor mention psychiatric disorders.
Mitterauer, 2010(18) is a review of the effects of connexin gap junctions from astrocytes in memory. Mitterauer, 2010 is focused on connexin gap junctions and does not mention connexin hemichannels from astrocytes.
Desplantez et al., 2012(19) studied the effect of a peptide in connexin 43 hemichannels. Desplantez et al., 2012 does not postulate this peptide as pharmacological agent for psychiatric disorders, nor a role for astrocytes in these pathologies.
Davidson et al., 2012(20) studied the use of a connexin 43 mimetic peptide for blocking the opening of connexin 43 hemichannels in an ischemia model. They postulate connexins as pharmacological targets for ischemia, but do not indicate that connexin hemichannels from astrocytes can be used as pharmacological targets, and do not mention treating psychiatric disorders.
Sun, et al., 2012(21) studied the effect of connexin 43 mimetic peptides on the dysfunction of connexin 43 gap junction in astrocytes and its relation with depression pathophysiology. Sun, et al., 2012 postulates astrocytes and connexin 43 gap junctions as pharmacological targets, but it is focused on connexin 43 gap junctions and does not mention connexin 43 hemichannels.
O'Carroll et al., 2008(22) studied blocking of connexin 43 hemichannels with mimetic peptides corresponding to intracellular loop of connexin 43 for demonstrating that this inhibition is useful to diminish propagation of cellular death in astrogliosis. O'Carroll et al., 2008 does not show any relationship between psychiatric disorders and modulation of connexin 43 hemichannels.
Ponsaerts et al., 2010(23) studied the functionality of connexin 43 mimetic peptides in vitro. The results show that this type of peptides regulates opening of connexin 43 hemichannels and not connexin 43 gap junctions. Ponsaerts et al., 2010 does not mention psychiatric disorders nor connexin 43 hemichannels from astrocytes.
US20100029613A1(24) describes a method to treat or prevent epileptic seizures administering agents that interfere with astrocytic release of glutamate, aspartate and/or ATP. A method is also presented to identify agents that may have such effects. US20100029613A1 finally tests all previously known glutamate release pathways to determine the mechanisms by which such release may occur. US20100029613A1 mentions the work of Ye et al. 2003 as previous art, which suggests a role of connexin 43 hemichannels in astrocytic glutamate release. US20100029613A1 does not interfere with the present invention as it is based on calcium and ATP dependent astrocytic release of glutamate, ATP and aspartate and shows that such release, which is used as a marker for their agent identification and anti-seizure effects, is not dependent on astrocytic connexin 43 hemichannels. In order to justify their negative results, the authors cite Ye et al., 2003(2) suggesting that “Cx hemichannels may have a role in pathological conditions such as ischemia and epilepsy”. In consequence, ischemia or epilepsy have been excluded from our invention, which only includes psychiatric disorders.
EP2344146A1(25) is related to a product comprising at least one connexin blocking agent and a psychotropic drug, to be used simultaneously, separately, or spread over time in patients suffering from psychiatric and/or neurodegenerative disorders. EP2344146A1 is not related to the present invention, since it discloses the use of blocking agents for neuronal gap junctions as adjuvants for improving treatment of different psychiatric and/or neurological disorders, and it does not consider either astrocytes or hemichannels.
EP2322149A1(26) discloses the use of a pannexin hemichannel inhibitor to treat white matter ischemia, periventricular leukomalacia, optic nerve ischemia, mild cognitive impairment and subcortical vascular dementia. EP2322149A1 does not interfere with the present invention, since it mentions participation of pannexin hemichannels from oligodendrocytes and it does not mention astrocytes, or connexins, or psychiatric disorders.
WO2006134494A2(27) describes methods and compositions to modulate connexin activity for treatment of cardiovascular, vascular, neurologic disorders, for treating wounds and other indications. WO2006134494A2 indicates that these compounds and methods could be used, for example, to reduce severity of adverse effects in diseases and disorders where localized disruption of direct cell-cell communication or prevention of hemichannel opening is desirable. WO2006134494A2 focuses on uses of anticonnexin compounds on neurological disorders, based on vascular effects of connexins in response to ischemia. It does not mention astrocytes, nor psychiatric disorders.
WO2007002285A2(28) discloses new methods for treating neurologic and psychiatric conditions. The methods comprise modulating production or activity of one or more substances that participate in signaling of calcium or release of glutamate from astrocytes, modulating the intracellular concentration of calcium in astrocytes, modulating expression or release of D-serine, or modulating expression or release of ATP or adenosine. Methods of detecting compounds with the ability to target to specific pathways or intracellular calcium are also disclosed. WO2007002285A2 focuses on modulation of astrocytic release of substances for the treatment of different disorders including psychiatric disorders and on the use of compounds that affect intracellular or extracellular regulation pathways for this release, for example, affecting calcium or adenosine levels, but they do not include release pathways, in particular, they do not mention connexin (43 or 30) or pannexin 1 hemichannels.